A squelch detector is typically used in a communication system having a high-speed serial interface, such as a Universal Serial Bus (USB) 2.0 system. In such implementations, a squelch detector operates to detect the presence of a differential communication signal on a serial interface communication channel. To conserve power, the unused component(s) in the serial interface channel is usually turned OFF or otherwise powered down if the squelch detector detects that there is no communication signal being sent or received. If the power level of the communication signal drops below a given threshold level, the squelch detector generates a detector output signal indicating that the channel is inactive, and the detector output signal is then used to power down the unused component(s). If the power level of the communication signal is greater than the given threshold level, the squelch detector changes the detector output signal to indicate an active status so as to power up the component(s) in the communication system.
In USB 2.0 systems, the squelch detector is typically implemented with a mixer stage and an amplifier stage. The mixer stage includes a differential offset biasing input pair of transistors and a differential input pair of transistors. The differential offset biasing input pair of transistors is coupled to two reference voltage levels (Vref and Vrefb), wherein the difference of the two reference voltage levels is the threshold voltage (Vth) used to determine whether the communication signal on the communication channel is active or inactive. The differential offset biasing input pair of transistors is coupled to the differential input pair of transistors to generate an output signal that is the subtraction of the differential threshold voltage (Vth=Vref−Vrefb) from the differential communication signal (Vin−Vinb). The mixer stage's output signal (Vin−Vinb)−Vth is sent to the amplifier stage so as to provide/amplify the detector output signal that indicates whether the differential communication signal is larger than the threshold voltage Vth. If the peak-to-peak potential of the differential communication signal is greater than the squelch detection threshold value (e.g., the threshold voltage Vth), the detector output signal will be high, thereby indicating that the communication signal is present on the communication channel. Vice versa, if the peak-to-peak potential of the differential communication signal is less than the threshold voltage Vth, the detector output signal of the squelch detector will be low, thereby indicating that the communication channel is inactive.
There are a number of drawbacks with the squelch detector described above. For example, random process variations on a chip can produce a mismatch of the differential pairs of transistors, and the mismatch in turn may cause an undesirable input offset voltage. The undesirable input offset voltage, which either adds to or subtracts from the two reference voltage levels Vref and Vrefb, leads to an inaccurate or an inadequate tripping voltage range for the squelch detector.
Moreover, the implementation of differential input pairs of transistors and the amplifier stage consumes a large amount of power due to static power used in current sources for biasing. An offset cancellation circuit may be implemented to counteract the undesired input offset voltage, but the offset cancellation circuit itself also consumes power.
Also, in certain serial interface protocols, like USB 2.0, a large input common mode range imposes a significant design challenge to keep all of the transistors in the squelch detector in saturation mode. From process to process, due to the change in transistor characteristics, a great amount of effort has to be spent in tuning the squelch detector and other circuitry to ensure the transistors are operating in the saturation region.